90

7 Energy Flow Through an Ecosystem

Now that you are familiar with producers and consumers,

you are going to learn about how these organisms interact

within an ecosystem. Picture a seal swimming in the Pacific Ocean

just off the coast of California. Chances are this seal is among a forest

of seaweed including a species called giant kelp. Giant kelp (Macro­

c ystis pyrifera) is a type of algae that grows up to two-thirds of a

meter a day, and over 45 m (about 148 feet) in height. At its base is

a woven knot of rootlike branches called a holdfast that attaches to

rocks on the ocean floor. The kelp grows in clusters with each plant

shooting upward to the surface, then spreading out and sheltering

thousands of organisms in a complex ecosystem.

The kelp forest ecosystem is often compared to an underwater

tropical rainforest, in part because of its high levels of biodiversity.

The kelp are tall, with long leaf-like structures that create a canopy

that blocks the light at lower ocean depths, providing habitat and

nourishment for organisms that thrive in limited light conditions.

Many species of aquatic organisms, such as sea urchins and ancho-

vies, live and reproduce on and among the long strands.

When organisms in the kelp ecosystem die, other organisms,

including specialized bacteria, consume their remains, keeping the

nutrients flowing through the ecosystem. These essential organisms

are called decomposers.

One way to show these interrelationships between the organisms in an ecosystem is

by creating a food web, mapping what each organism eats and how the energy flows

through an ecosystem. In the last activity the food web included plankton, herring,

and whales. In this activity you will construct a food web for a kelp forest ecosystem

and from it predict what will happen to the ecosystem in various circumstances.

ChallengeHow can we use food webs to predict the short- and long-term effects of particular 00

events on an ecosystem?

This harbor seal (Phoca vitulina) is swimming in a forest of giant kelp.

91

EnErgy Flow THrougH an EcosysTEm • acTiviTy 7

maTErials

For each group oF students

sheet of chart paper

set of colored pencils

For each pair oF students

set of 12 Kelp Forest Organism cards

set of 4 Ecosystem Event cards

ProcedurePart A: Construct a Food Web

1.With your group, read each Kelp Forest Organism card, noting the similari-

ties and the differences in what substances are going into and coming out of

each organism.

2. Sort the organism cards into at least two sets of cards based on similarities in

the information about the organisms.

3. As a group, develop a system for further sorting the organisms within each set of

cards. When your group comes to agreement, record in your science notebook

the organisms in each set of cards and the feature(s) they have in common.

4. Construct an ecosystem food web to show the direction of energy transfer

from one organism to another. Begin by laying the giant kelp card on the

table in front of your group.

Kelp forests are found off coastlines around the world.

Science&GlobaliSSueS/bioloGy • ecoloGy

92

5. Look through the cards and identify all of the organisms that feed on giant

kelp. Place these above the giant kelp card.

6. Continue placing cards on the table based on the organisms they feed on, form-

ing a food web. When you have placed all of the cards, show your teacher your

work.

7. With your group, record the food web on a piece of chart paper.

8. Draw arrows from one organism to the next to show how energy passes from

one organism to another.

Hint: Remember that arrows in a food web point in the direction of energy

flow—toward the organism that is doing the “eating,” as shown in the food

web below.

9. One way scientists classify organisms in an ecosystem is by describing how

the organisms obtain energy. This is known as the organism’s trophic role in

the food web. Read about these trophic roles in the table below.

10. Select a colored pencil to represent each of the trophic roles shown in the

table above. Make a key on the side of your chart paper to indicate which

color indicates which role.

Trophic Roles of Organisms in an EcosystemTyPE OF ORgAnism hOW ORgAnism gETs EnERgy TO susTAin liFE

Producer Transforms light energy or other energy sources into chemical energy

The chemical energy is stored in carbon-containing molecules, such as simple sugars or starch.

Consumer Feeds on other organisms to obtain energy

Decomposer Feeds on other organisms and dead and decaying biological material and wastes to obtain energy

93

EnErgy Flow THrougH an EcosysTEm • acTiviTy 7

11. With your group, color-code each of the organisms in the food web to indi-

cate its role in the ecosystem.

Part B: use a Food Web to Predict the impact of Actions and Events on an Ecosystem

12. Obtain a set of Ecosystem Event Cards from your teacher. With a partner,

select and read one of the cards.

13. Using the information provided by your kelp forest food web, discuss how

the event described on the card affects the ecosystem.

Hint: Choose one organism or factor in the web that is affected, and think of

the chain of events that will then occur throughout the ecosystem. Scientists

call this type of chain of events a trophic cascade.

14. Record the following for each event card in your science notebook:

a. summary of the event

b.what effect the event will have on the ecosystem in the

i. short term

ii. long term

c.what effect the event will have on the flow of energy through the kelp forest

ecosystem

Part C: Construct an Energy PyramidAn energy pyramid is a diagram of the amount of the sun’s energy that is stored in

each level of organisms in a food web. The organisms

use part of this energy, part of it is lost as heat, and part

of it is stored and therefore available to other

organisms.

15. With your group use the information from the

kelp forest food web to place the name of each

organism on the energy pyramid on Student

Sheet 7.1, “Kelp Forest Energy Pyramid.”

16. With the class discuss what the energy pyramid

shows.

This energy pyramid shows how much energy is stored at each level of the food web.

Science&GlobaliSSueS/bioloGy • ecoloGy

94

Analysis 1. Name one organism from the kelp forest ecosystem that is:

a producer

a consumer

a decomposer

2. What is the difference between the role of an organism that is a producer and

one that is a consumer? How is a decomposer different from other consumers?

3. Explain the role of the sun in the kelp forest ecosystem.

4.Describe the flow of energy in the kelp forest ecosystem.

5. From the lake food web at right:

a.predict what would happen if all of the walleye

were fished out of the lake

b.choose an organism other than the walleye,

and predict what would happen if it disap-

peared from the lake

6. Explain why a pyramid is helpful for describing

the amount of energy available in a food web.

7. Imagine you are an ecologist who studies kelp

forest ecosystems. You have been asked by the

federal government to evaluate two plans for

managing the California sheephead fishery,

which relies on kelp forests. A summary of two

proposals follows. Read the proposals and

determine:

a.how the two proposals differ

b.which proposal you think the government should implement

In your recommendation include a discussion of the trade-offs you

considered.

Proposals for managing the California sheephead Fishery

proposal a

Take population counts of California sheepheads once every three months.

If the fish population is below sustainable levels, decrease the size of the allowed

sheephead catch. If the sheephead population is at or above sustainable levels,

keep the catch limit at current levels.

Cost to implement this proposal: $750,000

lake food web

EnErgy Flow THrougH an EcosysTEm • acTiviTy 7

95

proposal b

Take population counts of California sheepheads and sea urchins once every three

months. Measure the density of the giant kelp population once every three months.

If the size of any population is below sustainable levels, decrease the size of the

sheephead catch allowed. If the size of each population is at or above sustainable

levels, keep the sheephead catch limit at current levels.

Cost to implement this proposal: $1,250,000

KEy vocabulary

biodiversity energy pyramid

consumers food web

decomposers producers